Oscillator circuit



ll""e b.'164 1943. fR. B. EDWARDS OSCILLATOR CIRCUIT Fild oct). 11. 1941I A .X20 I INVENTOR ROBERT E. E DWQRDS y ATTORNEY.

Patented Feb. 16, 1943 UNITED sTATEsPATENT OFFICE oscnaA'roR ontoUrrRobert n. Edwards, Baltimore, Md. Application october .11, 1941, serialNo. 414,683

8 Claims.

This invention relates to high frequency oscillators and moreparticularly to a high frequency oscillator which may be optionallypiezoelectric crystal controlled or continuously variable in outputfrequency.

Piezo-electric crystal controlled vacuum tube oscillators are well knownfor their frequency stability and their wide employment in thecommunications field is a direct result of this characteristic. Thereare times, however, particularly in military maneuvers, when it becomesdesirable, or even necessary, to employ transmission frequencies whichcould not be anticipated at the time of the design or the constructionof the transmitting equipment. Only an oscillator continuously variablein output frequency can meet such requirements for exibility, butunfortunately this type of oscillator cannot be designed to meet thestability requirements ofthe normal fixed channel operation. For themost part, equipment which has been constructed in the past has been acompromise, having the frequency either crystal controlled orcontinuously variable. Equipment incorporating both features has beenunduly heavy and bulky.

Attempts have been made to provide an oscillator circuit which, by asimple switching operation, may be adapted for operation at a fixedfrequency under crystal control, or for operation over a band as asource of continuously variable frequency. The most notable of theattempts made so far has failed of wide application because best crystalperformance and life require that the crystal itself oscillate at asubharmonic of the desired carrier frequency for transmission in theportion of the ether spectrum assigned to mobile craft, and, in thesystem referred to, the tuned output circuit must be tuned to, or verynear, the natural frequency of the crystal element. The principalcustomers for transmission equipment offering such savings in space andWeight as the systems mentioned are the owners of mobile craft and,since the above system did not meet their requirements, the market forequipment incorporating this system was correspondingly limited.

One of the principal objects of this invention is to provide a radiotransmitter having optionally crystal controlled fixed output frequencyor continuously variableoutput frequency and which is lighter, smaller,and cheaper than such previous transmitters.

Another object of this invention is to provide an oscillator optionallydelivering a fixed output A frequency under the influence of a highlystable frequency determining network or a continuously variable outputfrequency.

Still another object of this invention is to provide an oscillatoroptionally delivering a xed output frequency under crystal control whichis a harmonic of the natural frequency of the crystal control element ora continuously variable output frequency.

Other objects and advantages will in part be disclosed and in part beobvious when the following specification is read in conjunction with thedrawing, in which:

Figure 1 is a schematic diagram showing one form of the invention.

Figure 2 is a schematic diagram showing a modified form of theinvention.

Figure 3 is a schematic diagram showing a simplified form of theinvention.

It is to be understood that the drawing is intended to illustrate apreferred form of the invention, and is not to comprise a limitation onthe content or scope of the invention. y

In the drawing, like parts are designated by like reference characters.

Following the mention of a circuit element there is shown in parenthesisa value which has proven satisfactory in the practice of the inventionover the frequency range of two megacycles to seven megacycles.

Referring in greater detail to Figure 1, a vacuum tube I may be employedhaving, a cathode 2 which-is raised to a temperature suiiicient forthermal emission of electrons by the heater 3, a control grid 4, a spacecharge grid 5 and an anode 0. The commercial type 6V6 has beensuccessfully utilized in the practice of my invention. A choke 'I (2.5mh.), whose resistance provides control grid bias for the Vacuum tube Iis connected between the cathode terminal 2 and a ground 8, and acapacitor 9 (10 mmfd.) is connected from the cathode 2 to the controlgrid 4. The resistor In (25,000 ohms) provides a direct current pathfrom the control grid 4 to the ground 3 While the blocking capacitor I I(.01 mfd.) prevents any direct current voltage developed across theresistor I0 from being short circuited to ground upon closure of thesingle-pole, single-throw switch I3 which is connected, in parallel withthe piezoelectric crystal I2, between the blocking capacitor II and thelground 8.

The parallel resonant ycircuit comprising the tapped inductance I4,bypass capacitor I5 (.01 mid.)` and a variable tuning capacitor I6 (100mmfd.) is effectively connected in series with the anode circuit of thevacuum tube I, one terminal the parallel resonant circuit through theoutput coupling capacitor 23 (40 Inmfd.)

A voltage dropping resistor 20 (20,000 ohms) is connected from thepositive terminal of the source I1 to the space charge grid 5, and avoltage stabilizing resistor 2l (50,000 ohms) is connected between thespace charge grid 5 and the ground 8, the combination providing thedesired direct current potential on the space charge grid 5. Thecapacitor 22 (.003 mfd.) serves to maintain the cathode 2 and the spacecharge grid 5 at substantially the same alternating current potential.

Cperation of the oscillator with the switch I3 closed, thus effectivelygrounding the control grid 2 for high frequency currents, takes place inthe manner of the well known Colpitts Oscillator, for the cathode 2,istied to a tap on a part of the tuning capacitance of a resonant circuit.The frequency of oscillation may now be varied by adjustment of theinductance I4 and the tuning capacitor I 6.

With the switch I3 open, the mode of oscillation is considerablydifferent. It is Well known in the art that the control grid inputimpedance of a vacuum tube amplifier having a high capacitive reactancein the cathode lead has a considerable negative real component becauseof the capacitance existingbetween the cathode and the control grid.This negative resistance appearing across the piezo-electric element I2causes the element to oscillate, -generating piezo-electric voltages atthe natural frequency of the piezoelectric element. It is important tonote that with' practical values of circuit constants the cathodecircuit impedance will always be capacitive in nature and the crystal I2will continue to oscillate whatever the frequency to which the parallelresonant circuit is tuned. In fact, the parallel resonant circuit may beentirely removed and the crystal will still oscillate. For this reason,the parallel Vresonant circuit including inductance I4 and thecapacitances I5 and I6, which might also ibe termed the output circuit,may be tuned to any desired harmonic of the crystal frequency, so thatthe oscillator stage also serves as a frequency multiplier, eliminatingthe extra tube formerly required for this function.

Referring now to the modified form of the invention shown in Figure 2,several circuit elements have been rearranged in a manner not affectingtheir operation, and a few changes are shown which assist in themanufacture of this type of equipment using commercially availableparts.

The capacitor 9 is here connected from the space charge grid 5 to thecontrol grid 4, and the capacitor I8 is bridged from anode 6 to thespace charge grid 5. Since capacitor 22 is of sufficiently low impedanceto maintain the space charge grid 5 and the cathode 2 at substantiallyidentical 'alternating current potentials this part of the circuitoperates in the fashion previously described.

It frequently happens that the direct current resistance of commerciallyavailable chokes is not of the value giving the desired bias on thecontrol grid 4. In this case the direct -current return for the cathodecurrent may be provided by a circuit of the type shown, incorporatingthe choke 24 (2.5 mh.) and the resistances 25 (1500 ohms) and 25 (400ohms). The sum of the direct current resistances of 25 and 26 isselected to .provide the desired phase angle of the cathode load circuitand the choke 24 is bridged across a suiiicient portion of this totalresistance to secure the proper operating bias for the control grid 4.

The control grid 4 is connected to the movable member of a single-pole,double-throw switch 21, the fixed contacts of this switch beingconnected respectively to the blocking condenser II and t0 thepiezo-electric crystal I2. The other terminals of the two last-mentionedelements are connected together and to the ground 8.

With the switch 21 connected to the capacitor II, the oscillatoroperates as a Colpitts oscillator and the output frequency may becontinuously varied by adjustment of the inductance I4 and the tuningcapacitor I6. With the switch 21 connected to the crystal I2, thecrystal oscillates at its natural frequency and the inductance I4 andthe tuning capacitor I6 may be adjusted to select this frequency or anyreasonable harmonic of it, the crystal controlled frequency thusselected being fed to the desired load through the coupling capacitor23.

In Figure 3 is shown a simplified form of the invention utilizing thetriode type of electric discharge tube. The tube 21 including theemitting cathode 28 with its associated heater 29, has its control grid30 connected to the junction of the grid resistor I0 and the blockingcapacitor II` while the anode 3| is connected to one terminal of thetapped inductance I4. The circuit elements not associated with the spacecharge grid 5 of Figure 1 are retained in this form of the invention andare connected in substantially the same manner.

With the switch I3 in the closed position the circuit of Figure 3performs as a Colpitts oscillator whose operating frequency isdetermined by the resonance frequency of inductance I4 with thecapacitors I5, I 6, I8 and I9.

With switch I3 in the open position, the negative resistance appearingin the input circuit by virtue of the capacitive cathode load and thecapacitance 9 causes the piezo-electric crystal I2 to oscillate at itsnatural frequency, producing in the anode circuit of tube 21 alternatingcurrent components of the crystal frequency and the harmonics thereof.By properly tuning the parallel resonant circuit the desired harmonicmay now be fed into the output load.

In practice, it is obvious that the capacitor 9 may be dispensed withwhen employing tubes inherently having sufficient grid-cathode capacityfor the satisfactory operation of the circuit.

For the sake of simplicity, I have omitted the source of energy for theheaters from the drawing, but it is understood that this may be sup#plied according to the type of electric discharge device employed.

It will be obvious that many changes and modifications may be made inthe invention without departing from the spirit thereof as expressed inthe foregoing description and in the appended claims.

What I claim ls:

1. In a high frequency oscillator system, the combination of an electricdischarge device having a cathode, a control grid and an anode, aparallel resonant circuit comprising inductance and capacity, oneterminal of said circuit being connected to said anode, a capacitancevoltage divider connected in parallel with said circuit, a connectionbetween said cathode and an intermediate point on said voltage divider,an electromechanical resonator connected between said control grid andthe other terminal of said circuit, and means for disabling saidelectro-mechanical resonator and maintaining said control gridsubstantially at the alternating current potential of said otherterminal of said circuit.

2. In a high frequency oscillator system, the combination of an electricdischarge device having a cathode, a control grid and an anode, aparallel resonant circuit comprising inductance and capacity, oneterminal of said circuit being connected to said anode, a capacitancevoltage divider connected in parallel with said circuit, a connectionbetween said cathode and an intermediate point on said voltage divider,a capacitor effectively connected between said cathode and said controlgrid, an electro-mechanical resonator connected between said controlgrid and the other terminal of said circuit, and means for disablingsaid electro-mechanical resonator and maintaining sai-d control gridsubstantially at the alternating current potential of said otherterminal of said circuit.

3. In a high frequency oscillator system, the combination of an electricdischarge device having a cathode, a control grid and an anode, aparallel resonant circuit comprising inductance and capacity, oneterminal of said circuit being connected to said anode, a capacitancevoltage divider connected in parallel with said circuit, a connectionbetween said cathode and an intermediate point on said voltage divider,an electromechanical resonator, and a switch having a .plurality ofoperating positions, one operating position of said switch connectingsaid electro-mechanical resonator between said control grid and theother terminal of said circuit, and another operating position of saidswitch connecting cir- 4. Ina high frequency oscillator system, the

combination of an electric discharge device having a cathode. a controlgrid and an anode, a parallel resonant circuit comprising inductance andcapacity, one terminal of said circuit being connected to said anode, acapacitance voltage divider connected in parallel with said parallelresonant circuit, a connection between said cathode and an intermedi-atepoint on said voltage divider, a piezo-electric resonator connectedbetween said control grid and the other terminal of said circuit, andmeans for disabling said piezo-electric resonator and maintaining saidcontrol grid substantially at the alternating current potential of saidother terminal of said circuit.

5. In a high frequency oscillator circuit, the combination of anelectric discharge device having a cathode, a control grid an-d ananode, an inductance, one terminal of said inductance being connected tosaid anode, a capacitance voltage divider connected in parallel withsaid inductance, a connection between said cathode and an intermediatepoint on said voltage divider, an electro-mechanical resonator connectedbetween said control grid and the other terminal of said inductance, andmeans for disabling said electromechanical resonator and maintainingsaid control grid substantially at the alternating current potential ofsaid other terminal of said inductance.

6. In a high frequency oscillator system, the.

combination of an electric discharge -device having a cathode, -acontrol grid, a space charge grid and an anode, a parallel resonantcircuit comprising inductance and capacity, one terminal of said circuitbeing connected to said anode, a capacitance voltage divider connectedin parallel with said circuit, a connection between said cathode and anintermediate point on said voltage divider, means for maintaining saidcathode and said space charge grid at substantially the same alternatingcurrent potential, an electromechanical resonator connected between saidcontrol grid and the other terminal of said circuit, and means fordisabling said electro-mechanical resonator and maintaining said controlgrid substantially at the alternating current potential of said otherterminal of said circuit.

7. In a high frequency oscillator system, the combination of an electricdischarge device having a cathode, a control grid, a space charge gridand an anode, a parallel resonant circuit of inductance and cap-a-city,one terminal of said circuit being conn-ected to said anode, acapacitance voltage divider connected in parallel with said circuit, aconnection between said cathode and an intermediate point on saidvoltage divider, means for maintaining said cathode and said spacecharge grid at substantially the same valternating current potential,means for maintaining said space charge grid at a positive potentialwith respect to said cathode, an electro-mechanical resonator connectedbetween said control grid and the other terminal of said circuit, andmeans for disabling said electro-mechanical resonator and maintainingsaid control grid substantially at the alternating current potential ofsaid other terminal of said circuit.

8. In a high frequency oscillator system, the combination of an electricdischarge device having a cathode, a control grid, a space charge gridand an anode, a power source, a parallel resonant circuit connectedbetween the anode terminal of said power source and said anode, acapacitance voltage divider effectively connected in parallel with saidcircuit, a connection between said cathode and an intermediate point onsaid voltage divider, a capacitor effectively connected between saidcathode and said control grid, means for maintaining said cathoder andsaid space charge grid at substantially identical alternating currentpotentials, means for maintaining said space charge grid at a positivepotential with respect to said cathode, means having high impedance tooscillatory currents for completing a direct current circuit from saidcathode to the cathode terminal of said power source, a piezo-electricresonator connected between said control grid `and said cathode terminalof said power source, and means for disabling said piezo-electricresonator and maintaining said control -grid substantially at thealternating current potential of said cathode terminal of said source.

` ROBERT B. EDWARDS.

